The class 1 cephalosporinase (CepH) and class 2d oxacillinase (AmpH) from an Aeromonas hydrophila clinical isolate, strain T429125, have been cloned and sequenced. Both enzymes are typical of their equivalents in other species of Aeromonas Both cloned beta-lactamase genes were expressed at a low level in a standard laboratory Escherichia coli strain, but when cloned into a cre deletion E. coli mutant, they were expressed at significantly higher levels. Specific disruption of the creB gene resulted in similar increased levels of beta-lactamase expression, so it was concluded that CreB represses the transcription of ampH and cepH in a cre(+) E. coli strain. The expression of cepH was four times that of ampH in the deltacre mutant because of an additional factor encoded on the cloned T429125 chromosomal fragment containing cepH. This factor was able to trans-activate expression of co-resident ampH in the deltacre mutant such that expression of the two genes was approximately equal. The entire cepH-containing fragment was sequenced, but it contained no genes that were obviously related to any known class of DNA-binding protein.